Dear Editor,

please find enclosed a manuscript on Spectral Compression of Narrowband Single
Photons with a Resonant Cavity. This work develops the previous idea
time-lensing used in ultrashort pulse shape engineering into to a tool that
manipulates single photon states suitable for atom-photon interaction. We
demonstrate compression of heralded single photon states resonant with a
ground state transition in Rb-87, generated in four-wave mixing in a cold
cloud of Rb-78, from a bandwidth of 20MHz to less than 8 MHz, close to the
atomic transition. 

Contrary to the natural dispersion of fibers in the ultrashort pulse compression in earlier work on time-lenses, we utilize the dispersion of a half-sided optical cavity, as the bandwidth of light fields we consider are a few orders of magnitude smaller for light that can interact near resonance with single atoms. The technique is in principle lossless, making it a suitable tool for single photon manipulation for atom-light quantum interfaces.

In this first attempt, we can spectrally compress single photon states by a factor of almost 2.4. This idea may be useful for a wide range of physical systems and quantum interfaces between color centers in solids, molecules, quantum dots, and ions and atoms as carriers of quantum information may be couplet to each other.

We feel that the connection of ideas from ultrashort pulse physics to atomic physics and quantum information makes Physical Review Letters an ideal platform for this work, and ask you for your kind consideration of our manuscript as a letter.

With Best Regards on behalf of all authors,

Christian Kurtsiefer